Hereinafter, phases and angles are expressed in radians. The inventors of the present application have suggested a digital holography device including a phase-shifting array element that splits incoming light into four kinds of reference light beams with mutually different phases and emits the reference light beams (Patent Literature 1). FIG. 23 is an explanatory diagram showing the configuration of a conventional digital holography device 101. The digital holography device 101 includes a light source 104 that emits laser light. The laser light emitted from the light source 104 is split into laser light beams by a beam splitter BS. One of the separate laser light beams is reflected from a mirror M. The reflected laser light beam passes through a beam expander BE and is collimated by a collimator lens CL. The collimated light beam is projected to a subject 106 and reflected from the subject 106 to turn into object light beams. The object light beams pass through another beam splitter BS and then enter an image-capturing plane 100 provided in a CCD camera 103.
The other of the separate laser light beams into which the laser light has been split by the beam splitter BS is reflected from another mirror M. The reflected light beam passes through another beam expander BE and is collimated by another collimator lens CL. The collimated light beam is directed into an array device 102.
The array device 102 has four kinds of regions 107a, 107b, 107c, and 107d all of which are arranged in a checkerboard pattern in a plane perpendicular to a direction from which the laser light is directed into the array device 102. The four kinds of regions 107a, 107b, 107c, and 107d are arranged to respectively correspond to pixels of the CCD camera 103. The laser light beam having passed through the region 107a of the array device 102 becomes a reference light beam having a phase that serves as a reference phase for phase shift measuring means. The laser light beam having passed through the region 107b is converted into a reference light beam that is phase-shifted by −π relative to the phase of the reference light beam passing through the region 107a. The laser light beam having passed through the region 107c is converted into a reference light beam that is phase-shifted by −π/2 relative to the phase of the reference light beam passing through the region 107a. The laser light beam having passed through the region 107d is converted into a reference light beam that is phase-shifted by −3π/2 relative to the phase of the reference light beam passing through the region 107a. 
The four types of reference light beams generated by the array device 102 are reflected from still another mirror M. The reflected light beams pass through an imaging optical system, are reflected from another beam splitter BS, and then enter the image-capturing plane 100 provided in the CCD camera 103.
On the image-capturing plane 100 are recorded the four kinds of interference fringe patterns formed by interferences between the four kinds of reference light beams and the object light beams. An image reconstruction device 105 generates a reconstructed image of a subject 106 in accordance with the four kinds of interference fringe patterns recorded on the image-capturing plane 100.
Further, the technique of a single-shot phase-shifting digital holography by polarization wavefront segmentation is known (Non-Patent Literature 2). This arrangement uses a polarization imaging camera to concurrently record two interference fringe patterns whose phases are different from a phase of an object light beam on an image-capturing plane of the polarization imaging camera. Therefore, it is possible to obtain complex amplitude distribution of a stationary subject at one recording. However, it is impossible to obtain complex amplitude distribution of a moving subject since intensity distribution of the object light beam needs to be obtained each time the subject moves. Further, as compared with the arrangement shown in FIG. 23, this arrangement eliminates the need for the imaging optical system and eliminates the need for setting the optical system by micrometers. Moreover, the array device 102 is not used, causing no dependence upon wavelengths.
A phase distribution obtained by digital holography is folded into a range of −π<φ≦π. To get it back to the original phase distribution, it is necessary to perform phase unwrapping. As methods for the phase unwrapping, the followings have been suggested.
First, dual-wavelength phase unwrapping (Non-Patent Literature 5) is known. According to this method, it is possible to change a length of a synthetic wavelength to a desired length by virtue of a combination of two wavelengths. In addition, as compared with a single-wavelength phase unwrapping method, it is possible to obtain a phase distribution that is equal to a phase distribution obtained by recording with an extremely long synthetic wavelength, and a phase folding is small.
Further, the technique of parallel phase-shifting digital holography is known. This technique requires only one exposure for recording by virtue of spatial segmentation multiplexing, although other normal digital holography requires a plurality of shots for recording. Moreover, since a reconstructed image of the subject can be obtained instantaneously, the subject may be a moving object.
Patent Literature 1
    Japanese Patent Application Publication, Tokukai, No. 2005-283683 A (Publication Date: Oct. 13, 2005) Non-Patent Literature 1    Yasuhiro Awatsuji et al., “Parallel three-step phase-shifting digital holography”, 1 Mat 2006/Vol. 45, No. 13/APPLIED OPTICS, pp. 2995-3002Non-Patent Literature 2    Takanori Nomura et al., “Phase-shifting digital holography with a phase difference between orthogonal polarizations”, 10 Jul. 2006/Vol. 45, No. 20/APPLIED OPTICS, pp. 4873-4877Non-Patent Literature 3    X. F. Meng et al., “Two-step phase-shifting interferometry and its application in image encryption”, OPTICS LETTERS/Vol. 31, No. 10/May 15, 2006, pp. 1414-1416Non-Patent Literature 4    Yan Zhang et al., “Reconstruction of in-line digital holograms from two intensity measurements”, Aug. 1, 2004/Vol. 29, No. 15/OPTICS LETTERS, pp. 1787-1789Non-Patent Literature 5    Daniel Parshall et al., “Digital holographic microscopy with dual-wavelength phase unwrapping”, 20 Jan. 2006/Vol. 45, No. 3/APPLIED OPTICS, pp. 451-459